Transient increases in extracellular K+ produce two pharmacological distinct cytosolic Ca2+ transients

Transient increases in extracellular K+ are observed under various conditions, including repetitive neuronal firing, anoxia, ischemia and hypoglycemic coma. We studied changes in cytoplasmic Ca2+ ([Ca2+](cyt)) evoked by pulses of KCl in human neuroblastoma SH-SY5Y cells and rat dorsal root ganglia (DRG) neurons at 37degreesC. A "pulse" of KCl evoked two transient increases in [Ca2+](cyt), one upon addition of KCl (K-on(+)) and the other upon removal of KCl (K-off(+)). The K-on(+) transient has been described in many cell types and is initiated by the activation of voltage-dependent Ca2+ channels followed by Ca2+-evoked Ca2+ release from intracellular Ca2+ stores. The level of KCl necessary to evoke the K-off(+) transient depends on the type of neuron, in SH-SY5Y cells it required 100 mM KCl, in most (but not all) of dorsal root ganglia neurons it could be detected with 100-200 mM KCl and in a very few dorsal root ganglia neurons it was detectable at 20-50 mM KCl. In SH-SY5Y cells, reduction of extracellular Ca2+ inhibited the K. more strongly than the K-off(+) and slowed the decay of K-off(+). Isoflurane (I mM) reduced the K-on(+)- but not the K-off(+)-peak. However, isoflurane slowed the decay of K-off(+). The nonspecific cationic channel blocker La3+ (100 muM) had an effect similar to that of isoflurane. Treatment with thapsigargin (TG) at a concentration known to only deplete IP3-sensitive Ca2+ stores did not affect K-on(+) or K-off(+) suggesting that Ca2+ release from the IP3-sensitive Ca2+ stores does not contribute to K-on(+) and K-off(+) transients and that the thapsigargin-sensitive Ca2+ ATPases do not contribute significantly to the rise or decay rates of these transients. These findings indicate that a pulse of extracellular K+ produces two distinct transient increases in [Ca2+](cyt). (C) 2004 Elsevier B.V. All rights reserved.